"Our study demonstrates that exercise when young may reduce the risk of fractures later in life, and the old exercise adage of 'use it or lose it' may not be entirely applicable to the skeleton," said the study's principal investigator, Stuart J. Warden, assistant professor and director of research in physical therapy at the Indiana University School of Health and Rehabilitation Sciences at Indiana University-Purdue University Indianapolis (IUPUI).

Researchers exercised the right forearms of 5-week-old female rats for a few minutes three times a week for seven weeks. The left forearms were not exercised. Researchers did not exercise the rats for the next 92 weeks - virtually their entire lifespan.

“We knew that exercise increases bone size and strength, and that the skeleton is most responsive to exercise during the crucial growing years around puberty when you reach adult size and strength," Warden said. What was not known, however, was if the skeletal benefits of exercising while young would last a lifetime, Warden said. The study determined the answer to that question is, "Yes," Warden said.

The researchers found that the rats retained all of the skeletal exercise benefits they obtained while young even though they hadn't exercised for the rest of their lives, Warden said. "We found the exercise resulted in a lifetime increase in bone size in the right forearms of the rats and the bones of the left forearms never caught up in size," he said. How big a bone is determines how resistant it is to bending, or how strong it is, he said.

As humans age, bone loss occurs from the inside surface of the bone outward, Warden noted. Exercising while young lays down additional outside layers of bone. This results in a bigger bone than otherwise would be the case. "With more bone layers on the outside, you have more bone to lose," Warden said.